Method of forming an electrical resistance element



Sept. 23, 1969 J. G. CURTIS 3,468,011

METHOD OF FORMING 1m ELECTRICAL RESISTANCE ELEMENT Original Filed June 27, 1963 INVENTOR.

Jb hn G. Cari/s United States Patent Otfice 3,468,011 METHOD OF FORMING AN ELECTRICAL RESISTANCE ELEMENT John G. Curtis, Raleigh, N.C., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York Original application June 27, 1963, Ser. No. 291,190, now Patent No. 3,248,682, dated Apr. 26, 1966. Divided and this application Aug. 16, 1965, Ser. No. 479,794 Int. Cl. H01c 17/00 US. Cl. 29-620 2 Claims ABSTRACT OF THE DISCLOSURE A method of forming the film path of an electrically conductive film resistor. The film is formed into a plurality of paths which when unfolded flat form long narrow rhomboids of such size that the length of the long diagonals thereof are not substantially greater than that of the short diagonals, whereby current density at the ends of the film paths will be significantly reduced.

This application is a division of application Ser. No. 291,190, filed June 27, 1963, now US. Patent No. 3,248,682.

This invention relates to electrical resistance elements and more particularly to a method for modifying the film path of spiralled electrically conductive film resistors.

Such form of resistors comprises a linear dielectric body supporting a spiralled ribbon of resistance material along its length between bands of low resistance material supported on the ends of such body and serving as the resistance element terminals. The resistor is ordinarily formed of a substantially cylindrical rod of dielectric material bearing a resistance element comprising a spiral ribbon of carbon, carbon alloy, cermet, tin oxide, other metallic oxide, metal, or the like. To make such a resistor, a suitably coated cylindrical member is conventionally placed in a lathe and spirally grooved so that the coating material remaining is in the form of a spiral ribbon about the body between the terminals of low resistance material.

It has been observed that failure occurs in such a resistor in localized regions in the margins of the coating adjoining one or the other of the terminals.

From an investigation made, by laying out such a ribbon and its terminals in a flat plane, it has been discovered that the reason for such failure is that the outline of the ribbon between its terminals comprises a rhomboid; thus the shortest path between the terminals is a straight line extending diagonally across such a rhomboid between its wide angle corners, resulting in high current concentration along such line at such corners. Exact regions of failure are thus pinpointed as being the corners at the ends of the short rhomboid diagonal.

One solution to this problem is disclosed in United States Patent No. 2,910,664, issued to I. G. Lanning, wherein the bands of low resistance material, serving as the resistor element or ribbon terminals, are extended transversely of the ribbon ends at an angle of 90 so that the current between such terminals is uniformly distributed throughout the ribbon width along its entire length. This solution, although satisfactory, is sometimes impractical in practice because it requires an extra firing step for the bands of low resistance material. Furthermore, it is not suitable for resistors having ribbons of very high pitch since a large portion of the ribbon must be rendered inactive for resistance purposes when applying the low resistance material.

Patented Sept. 23, 1969 It is an object of this invention to provide a resistance element which overcomes the heretofore noted difficulties, and to provide a conductive film resistor having a substantially uniform path between its terminals through its resistive portion.

In accordance with the present invention the resistive portion of the resistor is divided into a plurality of paths such that when laid out in a flat plane, the diagonals of each rhomboidal path are not significantly different in length thereby providing a substantially linear rectangular current path. The plurality of paths so formed also distribute the electric current so that no one path will be subjected to current densities which will cause failure. Further, in accordance with the instant invention, a resistor may be suitably formed for all values of path pitch.

Additional objects, features, and advantages of the present invention Will become apparent, to those skilled in the. art, from the following detailed description and the attached drawing on which, by way of example, only the preferred embodiment of this invention is illustrated.

FIGURE 1 is a side elevation of a resistor of conventional form.

FIGURE 2 is a diagrammatic view showing the ribbon coating and terminals of a conventional form of resistor laid out in a single plane.

FIGURE 3 is a diagrammatic view corresponding to FIGURE 2 illustrating the instant invention.

FIGURE 4 is a side elevation, similar to FIGURE 1, of a resistor embodying the instant invention.

In the resistor illustrated in FIGURE 1, the dielectric member 10 has low resistance metal bands or terminals 12 and 14 formed thereon at its ends, with an electrically conductive coating 16 of resistance material in ribbon form spiralled about said member 10 between said bands 12 and 14. As seen in FIGURE 2, when the respective terminals 12 and 14 and the coating 16 are laid out in a single plane, coating 16 takes the form of a rhomboid. Accordingly, the shortest path between terminals 12 and 14 is a line 18 drawn diagonally across coating 16 between the wide angled corners 20 and 22, resulting in high current concentration at such corners.

Referring now to FIGURES 3 and 4, it is seen that the above faults are overcome by providing a plurality of paths 24 between the low resistance terminals or bands 26 and 28. The short rhomboid diagonal of each path, illustrated by the numeral 30, is not significantly different from the long diagonal of each said path, thereby forming a plurality of substantially rectangular paths. The electric current will now flow substantially uniformly through the plurality of substantially parallel paths provided, reducing the current density to which each said path is subjected to a much lower value, thereby preventing failure as hereinabove described.

It has been found that a particularly suitable and preferable resistor construction, not subject to the hereinbefore noted failure, may be obtained by maintaining a ratio of approximately 1:30 between the width and the length of the substantially rectangular paths thereof.

Although the present invention has been described with respect to specific details of certain embodiments thereof, it is not intended that such details be limitations on the scope of the invention except insofar as set forth in the following claims.

I claim:

1. In the method of forming an electrical resistance element comprising a dielectric member of substantially cylindrical form having an electrically conductive coating on the surface thereof and low resistance terminals at the ends thereof, the improvement comprising dividing said 3,468,011 g 3 4 electrically conductive coating into a plurality of paths References Cited where the unfolded flat paths form long narrow rhornboids of such size that the length of the long diagonals UNITED STATES PATENTS thereof are not substantially greater than that of the short 2,503,418 4/1950 scrantom X diagonals, the width to length ratio of the paths being at 2,910,664 10/1959 Lanmngg 338 300 least about 1:30, whereby the current density at the ends 5 3,311,968 4/1967 Ardoum et 29-4557 X of the film paths will be significantly reduced, wherein all said paths extend from one of the end terminals to the JOHN CAMPBELL Pnmary Exammer other and all said paths are in contact with both said JOHN L. CLINE, Assistant Examiner terminals at each respective end of said member.

2. The method of claim 1 wherein the ratio of the width 10 US. Cl. X.R. to the length of said paths is approximately 1:30. 29-621; 338308 

